IS-IS Control Plane Extensions

The following CLI enables SRv6 in the IS-IS instance and assigns a locator to each algorithm (zero or flexible algorithm).

configure
+--router
+---isis <0..31>
|   +---segment-routing-v6
|   |   +---locator <locator-name>
|   |   |   +--level-capability <level-1|level-2|level-1/2>
|   |   |   +---level <1|2>
|   |   |   |   +---metric <1..16777215>
|   |   |   +---tag <1..4294967295>
|   |   +---shutdown
|   |   |   no shutdown

The IS-IS control plane extensions in support of SRv6 are defined in draft-ietf-lsr-isis-srv6-extensions.

The IS-IS control plane advertises the SRv6 capabilities sub-TLV and the SRv6 Locator TLV. This includes the End function sub-TLV (equivalent to the prefix SID sub-TLV in SR-MPLS) and the End.X function sub-TLV (equivalent to the adjacency SID sub-TLV for a P2P link and a LAN in SR-MPLS).

The weight field in the End.X or LAN End.X sub-TLVs is not filled in on transmit and is ignored on receipt of the link TLV.

Table 1 desribes the supported IS-IS SRv6 TLVs in SR OS.

Table 1. SRv6 IS-IS TLVs
SRv6 TLV/Sub-TLV	Codepoint	IS-IS Context TLV	Description	SR OS Support
SRv6 Capabilities sub-TLV	25	Router CAPABILITY TLV (242)	Indicates SRv6 support	Yes
SR-Algorithm sub-TLV	19	Router CAPABILITY TLV (242)	Indicates base algorithm 0 and Flex-Alogo 128-255 support	Yes
Maximum Segments Left MSD Type sub-TLV	41	Router CAPABILITY TLV (242)	Indicates how deep a node terminating current segment can process the SRH (Segments-Left field) to move the next SID to outer IPv6 header DA field	Yes (advertised value = 11 SIDs) Received TLV is displayed in Link State DB but is not used for any purpose.
Maximum End Pop MSD Type sub-TLV	42	Router CAPABILITY TLV (242)	Maximum number of SIDs in a SRH when a node removes SRH (PSP or USP modes of SRH)	Yes (advertised value = 11 SIDs) Received TLV is displayed in Link State DB but is not used for any purpose.
Maximum H.Encaps MSD type sub-TLV	44	Router CAPABILITY TLV (242)	Indicates maximum number of SIDs in an SRH a router can push when forwarding a IP or L2 packet over a SRv6 policy	Yes (advertised value = 1 SID) Received TLV is displayed in Link State DB but is not used for any purpose.
Maximum End D MSD Type Sub-TLV	45	Router CAPABILITY TLV (242)	Maximum number of SIDs in a SRH when a node removes SRH and performs the End.DX2/4/6 or End.DT4/6 function (USP mode of SRH)	Yes (advertised value = 8 SIDs) Received TLV is displayed in Link State DB but is not used for any purpose.
SRv6 Locator TLV	27	Is a top Level IS-IS TLV	Advertises the locator prefix configured on this node to terminate SIDs in algorithm 0 and flex-algo 128-255	Yes
SRv6 End SID sub-TLV	5	SRv6 Locator TLV	Advertises the SID for the endpoint or End function (equivalent of prefix SID sub-TLV in SR-MPLS)	Yes
Prefix Attribute Flags Sub-TLV	4	SRv6 Locator TLV (also in IP Reach TLV 236)	Provides attributes of a prefix which is leaked between IS-IS levels	Yes
SRv6 End.X SID sub-TLV	43	Top level Extended IS reachability TLV (22)	Advertises the SID for the adjacency over a P2P link (equivalent of adjacency SID sub-TLV for P2P link in SR-MPLS)	Yes
SRv6 LAN End.X SID sub-TLV	44	Top level Extended IS reachability TLV (22)	Advertises the SID for the adjacency over a LAN (equivalent of adjacency SID sub-TLV for LAN in SR-MPLS)	Yes
SRv6 SID Structure Sub-Sub-TLV	5	SRv6 End SID Sub-TLV, SRv6 End.X SID Sub-TLV, SRv6 LAN End.X SID Sub-TLV	Provides the length of each field (Block, Locator, Function, and Argument) of the SRv6 SID it is advertised with	No SR OS does not advertise this sub-sub-TLV. If received from other vendor's implementation, it is not displayed in Link-State database and is also not propagated with the locator TLV.

When both SR-MPLS and SRv6 are enabled on the same IS-IS instance, an MPLS node SID cannot be configured for a prefix of the locator or an End SID. This is because the SRv6 locator subnet cannot be added to a network interface and an MPLS node SID is configurable against a network interface only.

However, both the SRv6 locator route and the SR-MPLS tunnel are programmed if IS-IS receives from a third-party router implementation, a /128 prefix that has both a locator TLV and a prefix SID TLV (with node flag enabled). If the prefix SID is for a subnet larger than /128, only the locator route is programmed and the SR-MPLS tunnel is not.

Each SID function encoded in a SRv6 SID has its own endpoint behavior codepoint as listed in Table 2.

Note: SRv6 standards provide the flexibility to advertise transport and service SIDs in both IS-IS and BGP. In SR OS, the IS-IS control plane only advertises the transport SID functions shown in Table 2 and only uses the transport SID for building the LFA repair tunnels.

SR OS advertises service SID functions in the BGP control plane only as described in BGP Service control plane extensions.

For the SRH processing and removal at the SID termination , the following modes of operation are associated with the termination of the End or End.X SID. These modes are sometimes referred to as SID flavours and IS-IS assigns a unique codepoint for each mode of the same End or End.X SID.

Basic or unflavored mode: The router that terminates an End or End.X SID and the Segments-Left field in the received packet is 0 before decrementing, keeps the SRH in the packet, and processes the packet identified by the next-header in the SRH.Typically, the next-header indicates another SRH and the packet is then forwarded based on the lookup of that next-SID; SR OS does not support this mode.
Ultimate SRH Popping (USP) mode: The egress PE terminates the last End or End.X segment in the outer IPv6 header and then processes the service SID in the SRH. SR OS supports this mode.
Penultimate SRH Popping (PSP) mode: The router that terminates the End or End.X segment before the last in the segment list, meaning the Segments-Left field before decrementing has a value of 1, removes the SRH on behalf of the egress PE. SR OS supports this mode.
PSP&USP mode: This is a combination of both the USP and PSP modes. The router that terminates the End or End.X segment applies the corresponding behavior for value 0 and 1 of the Segments-Left field. SR OS does not support this mode.

Ultimate Segment Decapsulation (USD) mode: This is a variant of the USP in which the node skips the SRH and moves directly to process the next header indicated in the SRH. SR OS does not support this mode.

Table 2. SRv6 SID Function Endpoint Behavior Codepoints
SID Function Endpoint Behavior	Codepoint RFC 8986	SID Type: End.SID	SID Type: End.X SID	SID Type: LAN End.X SID	Advertising Protocol	Supported
End (PSP, USP, USD)	1-4, 28-31	Yes	No	No	IS-IS	Yes IS-IS only {PSP value=2, USP value=3}
End.X (PSP, USP, USD)	5-8, 32-35	No	Yes	Yes	IS-IS	Yes IS-IS only {PSP value=6, USP value=7} }
End.T (PSP, USP, USD)	9-12, 36-39	Yes	No	No	IS-IS	No ⁽¹⁾
End.DX6	16	No	Yes	Yes	BGP or IS-IS	No ⁽¹⁾
End.DX4	17	No	Yes	Yes	BGP or IS-IS	No ⁽¹⁾
End.DT6	18	Yes	No	No	BGP or static	Yes⁽¹⁾ BGP only.
End.DT4	19	Yes	No	No	BGP or static	Yes⁽¹⁾ BGP only.
End.DT46	20	Yes	No	No	BGP or static	Yes⁽¹⁾ BGP only.
End.DX2	21	Yes	No	No	BGP or static	No⁽¹⁾ BGP only.

Note:

IS-IS saves SID sub-TLVs for endpoint behavior values that it does not support if received from a third-party implementation. However, it only uses End and End.X endpoint behaviors in RLFA and TI-LFA.

BGP advertises the supported endpoint behaviors End.DT4, End.DT6, End.DT46, and End.DX2 and accepts any behavior codepoint as long as the NLRI type is supported.